Fog in the pass

Several of you have asked about the fog in the pass...they thought the mountains should be clear due to their higher elevations...and in fact they have been clear for several days. The cams in Snoqualmie Pass (see attached) do show fog. In fact, the fog has been freezing on trees and other objects. Yet on top of tiger mountain and at Paradise it is sunny...what's up?

The explanation is that eastern Washington is full of low clouds and fog (as it frequently is under high pressure in the winter). They have an inversion overhead. The pressure is higher in eastern Washington than western and this difference is increasing as large scale high pressure is moving eastward. The six hour forecast valid at 10 AM this morning shows the story..cold air in eastern Washington and high pressure there, with a big pressure difference across the Cascades (roughly 10 mb..which is a lot). This difference in pressure is producing strong easterly winds in the Columbia Gorge. It is also pushing the cold air and fog up the eastern slopes of the Cascades and into the passes. You can see this in the visible satellite picture this morning (see picture).

Today, was a good example of a weakness in our current computer models...they failed to produce the low clouds of last night and this morning. We believe that an important reason for this failure is that the models mix the atmosphere too much under stable conditions (like when cold dense air is under warmer, less dense air). The simulation for today at 10 AM is a good example (see below--no clouds over PS).
Today, a good portion of Puget Sound lost the low clouds and we finally saw some sun. One reason is the inversion became so shallow that limited solar radiation could mix it out. Also the strong descending easterlies aloft (forced by the large difference in pressure) might have contributed. WIth solar energy reaching the surface, temps warmed into the mid-forties to 50F. The inversion weakened a bit...but is still there...and will restrengthen tonight as temps cool near the surface. Tomorrow morning I will try tiger mountain..with a thermometer, and see what I find.

PS: Just a reminder...I will be talking about NW snowstorms on Wednesday at 7 PM at Third Place Books in Lake Forest Park.
Will review big historical lowland storms, including the last event, and the rare conditions that produce them.